Certain metals shrink upon solidication from the liquid state. If unchecked this property leads to the formation of undesirable internal cavaties or pipe when such metals are formed or poured into foundry castings or primary ingots. As is well-known in the art, it is desirable to provide a reservoir of molten metal above a casting or ingot to feed the void created as the body of the casting or ingot solidifies. This reservior is known as a riser in the production of foundry castings and is referred to as a hot top in the production of primary ingots. A further function of the riser or hot top is to act as a collector for the chemical segregates, such as carbon and sulfur in the case of ferrous metals, which tend to stay in the liquid phase and accumulate in the area of final solidification. It is common practice to provide a covering of exothermic or insulating powder over the riser or hot top to minimize heat loss through the upper surface. These powders are commonly referred to as hot topping compounds.
Traditionally, people have tended to use exothermic materials for hot topping. These exothermic materials ignite, burn and give off heat. After the exothermic hot topping has completed it's burn, it leaves an insulating residue. The exothermic materials are generally made from aluminum dross which is of inconsistant composition and contains a variety of impurities. These exothermic hot topping materials will smoke when burning which is objectionable, and may also give off toxic and pungent fumes. The insulating hot topping materials (non-exothermic) have been used to a lesser extent. Materials such as perlite, vermiculite, rice hulls, wheat hulls, etc., have all been used as insulating hot top materials. But these materials are relatively inefficient insulators so that the largest part of the market has been exothermic.